The present invention relates to a power transmission mechanism capable of absorbing a torque change transmitted between two rotating members.
Japanese Unexamined Patent Publication No. 10-267045 discloses an example of a power transmission mechanism. As shown in FIG. 11, a pulley 101 functions as a first rotating member at an external drive source side, and a hub 102 functions as a second rotating member at an equipment side. The pulley 101 and the hub 102 are operably connected through an elastic member 103 that is made of rubber and is sandwiched between engaged recesses 104 formed on the pulley 101 and the hub 102.
When drive power is transmitted from the external drive source to the equipment, the elastic member 103 is deformed due to torque transmitted between the pulley 101 and the hub 102, which allows relative rotation between the pulley 101 and the hub 102. At the same time, the torque allows the relative rotation angle between the pulley 101 and the hub 102 to vary within a predetermined range. As a result, the relative rotation between the pulley 101 and the hub 102 absorbs a differential torque transmitted between the pulley 101 and the hub 102 whenever the transmitted torque between the pulley 101 and the hub 102 changes due to changes in output torque from the external drive source and/or changes in torque in the torque receiving equipment itself.
Japanese Unexamined Patent Publication No. 10-267045 discloses only a torque limiting function of a drive power transmission apparatus as shown in FIG. 11. The elastic member 103 disengages from the engaged recess 104 thereby disconnecting the power transmission between the pulley 101 and the hub 102, when a drive torque acting on the equipment becomes excessive. However, it is apparent that the torque change disclosed in the above publication is absorbed by optimizing the elastic coefficient of the elastic member 103 corresponding to a slope of a line 211 in FIG. 12 as will be later described.
In a power transmission mechanism according to the above publication as shown in FIG. 11, the elastic member 103 has a cylindrical shape. Also, a recessed curved surface 104a having the same curvature as a cylindrical surface 103a of the elastic member 103 is formed in a recess 104. In other words, the recessed curved surface 104a of the engaged recess 104 contacts the elastic member 103 entirely, when the transmitted torque is zero.
To increase the relative rotation angle between the pulley 101 and the hub 102 and to counter the large elasticity of the elastic member 103, the elastic member 103 must be deformed sufficiently even near a zero degree state of the relative rotation angle. As a result, FIG. 12 shows a graph between the transmitted torque of the power transmission mechanism and the relative rotation angle according to the prior art. A straight line 211 has a somewhat large slope. Therefore, the transmitted torque between the pulley 101 and the hub 102 changes suddenly within a predetermined range of angle based on the changes in the relative rotation angle.
To simplify the description of the above problem derived from a character illustrated in FIG. 12, the conventional power transmission mechanism assumes that the equipment such as the hub 102 rotates at a constant speed while the pulley 101 rotates due to the torque changed by the external drive source. That is, the torque change is accompanied by the change of the relative rotation angle. In this case, a change in rotation angle or in torque at the external drive source is defined as relative rotation angle of the pulley 101 with respect to the hub 102.
Also assume the following state. As indicated with dotted lines 212 in FIG. 12, a torque is caused by a compact equipment or an equipment whose torque is variable is in a small drive torque state, that is, in a state of a small relative rotation angle between the pulley 101 and the hub 102. In addition, a large change in a relative angle indicated by a curve 213 generates between the pulley 101 and the hub 102 at the external drive source. In this case, around troughs of torque generated by the external drive source, which is indicated by peaks on the left of the curve 213, the pulley 101 rotates relative to the hub 102 in a reverse direction with respect to a state in which the torque is zero, which is reverse to a normal direction, which transmits the torque from the external drive source to the equipment.
The reverse torque amplifies the differential torque generated by the power transmission system between the hub 102 and the equipment, which is indicated with the curve 214 as shown in FIG. 12. The excess torque load, substantially represented by negative half of the curve 214 of differential torque, generates a torque that works in a reverse direction or a negative torque. If there is a clearance in a torque transmitting direction between the hub 102 and the equipment or inside the equipment, relative rotation between the rotating members is caused and the power transmission in a normal direction is interrupted. The positive and negative torque act relatively and alternately to each other to offset the clearance of the rotating members. The clearance causes abnormal vibrations and noise due to collision of the rotating members, and it eventually wears the rotating members and increases chattering.
The present invention provides a power transmission mechanism which is capable of absorbing a change of a torque transmitted between a first rotating member and a second rotating member during a power transmission process. It also provides a power transmission mechanism which reduces a negative torque to an equipment even if a large torque change occurs at the external drive source.
To achieve the above objectives, the present invention has following features. A power transmission mechanism has a first rotating member rotating around an axis, a second rotating member rotating around the axis and an elastic member located between the first rotating member and the second rotating member. The first rotating member has a first rotating member surface, and the second rotating member has a second rotating member surface. The elastic member has at least a protrusion contacting at least the first rotating member surface. The protrusion extends in a circumferential direction of the first rotating member. The protrusion has gradually tapering shape toward the first rotating member surface. The elastic member is deformed by compression as the elastic member engages the first rotating member surface and the second rotating member surface by rotation of the first rotating member, thereby causing relative rotation of the first rotating member and the second rotating member.